1. Field of the Invention
The present invention generally relates to a method for controlling the air/fuel ratio of a fuel supply system for a internal combustion engine.
2. Description of Background Information
In order to supply a proper amount of fuel to an internal combustion engine, a fuel supply system is known in which a basic fuel supply amount is calculated by basic engine parameters, such as a pressure in the intake passage of the engine, in synchronism with the engine rotation. To derive an actual fuel supply amount, an increment or decrement of compensation is applied to the basic fuel supply amount in response to auxiliary engine parameters, such as an engine coolant temperature, or a parameter indicative of a transitional change of the engine operation. The fuel is supplied to the engine using a fuel supply device such as a fuel injector or injectors during time periods each of which corresponds to the actual fuel supply amount derived in the above-mentioned manner.
In the case of this type of fuel supply control system, if a three-way catalytic converter is provided in an exhaust system of the engine for the purification of the exhaust gas, the operation of the three-way catalytic converter is optimized when the air/fuel ratio of the mixture is controlled at around a stoichiometric value (14.7:1, for instance).
For satisfying this requirement, an arrangement is generally utilized in which oxygen concentration in the exhaust gas is detected as one of the engine parameters by means of an oxygen concentration sensor (abbreviated as O.sub.2 sensor hereinafter) provided in the exhaust system. The basic fuel supply amount is corrected in accordance with an output signal of the O.sub.2 sensor so as to effect a feedback control operation through which the air/fuel ratio of the mixture supplied to the engine is controlled to the stoichiometric value.
However, feedback control of the air/fuel ratio is not always effected. During a predetermined operating condition of the engine such as in a state where the engine coolant temperature is low, or during a high load operating condition of the engine, the air/fuel ratio is enriched by an open loop control where the air/fuel ratio is determined irrespective of the output signal of the O.sub.2 sensor.
In addition, in this type of fuel supply control system, the air/fuel ratio is enriched by increasing the fuel supply amount when the engine is operating under a high load condition. Because it is inconvenient if the feedback control of the air/fuel ratio is performed during the period of fuel increment control, a control method is disclosed in Japanese Patent application laid open No. 59-548 in which, when the fuel supply amount exceeds a predetermined value, it is detected that the engine is operating under a high load condition and the open loop control is selected instead of the feedback control of the air/fuel ratio.
However, if the high load condition is determined from the fuel supply amount and the air/fuel ratio is enriched when the fuel supply amount becomes greater than the predetermined value, the air/fuel ratio is also enriched for a short-time during acceleration of the vehicle. Such an enrichment of the air/fuel ratio will result in an increase of the emission of carbon monoxide (CO), to reduce the efficiency of purification of the exhaust gas.